The present invention relates to a light source device for use in an image forming apparatus and including a semiconductor laser, particularly a plurality of semiconductor lasers.
A light source device including a semiconductor laser and a collimator lens is extensively used in a digital copier, laser printer, facsimile apparatus or similar image forming apparatus. As for optical characteristics, the light source device is required to have the directionality (optical axis characteristic) and parallelism (collimation characteristic) of a laser beam to issue from the device. To satisfy such characteristics, it is a common practice to adjust the emission point of the laser and the collimator lens relative to each other in the directions of three axes x, y and z. The required positional accuracy is less than the order of micron. Therefore, the light source device of the type described should be adjustable in position in the three directions x, y and z and should be fixed in place at its adjusted position.
Adhesive used to affix the collimator lens contracts during curing. It is therefore necessary to reduce the adverse influence of the contraction on the optical characteristics. Particuarly, the accuracy of the light source device is severely restricted in the direction z (optical axis direction), so that the device must be so constructed as to obviate the contraction in the direction z. It is therefore preferable that the adhesive layer be substantially parallel to the optical axis or z axis, and that the contraction be limited to one of the x axis direction and y axis direction in order to facilitate adjustment.
Light source devices using a semiconductor laser and a collimator lens are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 5-88061, 5-136952, and 5-273483. The conventional light source devices, however, have some problems left unsolved, as follows.
(1) The devices are expensive because they need a number of parts.
(2) Displacements occur in the three directions x, y and z during assembly, lowering the accuracy of directionality of the laser.
(3) To affix the collimator lens, use cannot be made of photo-curable adhesive capable of curing rapidly in a desired configuration and having high reliability.
On the other hand, a multibeam scanning device capable of scanning a photoconductive element with a plurality of laser beams is available with a digital copier, laser printer or similar image forming apparatus. This type of scanning device includes a plurality of semiconductor lasers arranged in the subscanning direction. Laser beams issuing from the lasers are so combined as to lie on optical axes adjoining each other, and then output in one direction. A light source device for use in such a scanning device is, of course, required to have an accurate beam pitch, i.e., distance between the laser beams in the direction y.
Light source devices for emitting a plurality of laser beams are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-181410 and 7-181412. However, the devices taught in these documents have the following problem (4) in addition to the problems (1)-(3).
(4) The beam pitch of the semiconductor lasers cannot be accurately maintained due to the variation of temperature around the light source device.
The conventional light source devices have problems to be discussed later in addition to the problems (1)-(4).
It is therefore an object of the present invention to provide a cost-effective and accurate light source device needing a minimum number of parts, obviating displacements during assembly, and allowing collimator lenses to be affixed by photo-curable adhesive.
It is another object of the present invention to provide a light source device allowing a minimum of variation to occur in the distance between semiconductor lasers in the beam pitch direction even when temperature around the device varies.
In accordance with the present invention, a light source device includes a base formed with a plurality of through bores. A plurality of semiconductor lasers are positioned at the rear side of the base, and each is received in the respective through bore of the base. A plurality of collimator lenses are respectively adhered to a plurality of lens support portions formed on the front side of the base. The collimator lenses each is positioned coaxially with the optical axis of the respective semiconductor laser. A plurality of apertures each shapes a laser beam to issue from the respective collimator lens. A beam combining optical element combines laser beams to respectively issue from the semiconductor lasers to thereby output laser beams lying substantially on a single optical axis. The lens support portions each has a center line extending substantially perpendicularly to a beam pitch direction of the laser beams output from the collimator lenses.
Also, in accordance with the present invention, a light source device is made up of a plurality of semiconductor lasers, a base formed with through bores for respectively press-fitting the semiconductor lasers, a plurality of lens support portions formed on the base, a plurality of collimator lenses respectively adhered to the lens support portions via adhesive layers, an optical element for combining laser beams output from the collimator lenses to thereby produce beams adjoining each other, and a case mounted to the base for covering the collimator lenses and optical element. The adhesive layers each has a center thereof shifted outward of the respective bore in a beam pitch direction, so that the adhesive layers each thermally expands inward.